1. Field of the Invention
The present invention relates to measurement of electrical characteristics of tissue subjected to electrical stimulation by a cochlear implant.
2. Related Art
Cochlear implant systems bypass the hair cells in the cochlea and directly deliver electrical stimulation to the auditory nerve fibers, thereby allowing the brain to perceive a hearing sensation resembling the natural hearing sensation normally delivered to the auditory nerve. U.S. Pat. No. 4,532,930, the content of which is incorporated herein by reference, provides a description of one type of traditional cochlear implant system.
Cochlear implant systems have typically consisted of an external component referred to as a processor unit and an internal implanted component referred to as a stimulator/receiver unit. These components cooperate to provide the sound sensation to a user.
The external component has a microphone for detecting sounds, such as speech and environmental sounds, a speech processor that converts the detected sounds into a coded signal, a power source such as a battery, and an external transmitter coil.
The coded signal output by the speech processor is transmitted transcutaneously to the implanted stimulator/receiver unit situated within a recess of the temporal bone of the user. This transcutaneous transmission occurs via the external transmitter coil which is positioned to communicate, generally via RF, with an implanted receiver coil provided with the stimulator/receiver unit. This communication serves two purposes, firstly to transcutaneously transmit the coded sound signal and secondly to provide power to the implanted stimulator/receiver unit.
The implanted stimulator/receiver unit includes a receiver coil that receives the coded signal and power from the external processor component, and a stimulator that processes the coded signal and outputs a stimulation signal to an intracochlea electrode assembly or array. The electrode array applies the electrical stimulation directly to the auditory nerve producing a hearing sensation corresponding to the original detected sound.
Following implantation, and at subsequent times as physiological changes occur, there is a need to determine the actual performance of the electrode array and the response of the auditory nerve to stimulation. Data collection for this purpose enables detection and confirmation of the normal operation of the device, and allows stimulation parameters to be optimized to suit the characteristics of the patient. This procedure can include determination of patient specific parameters such as threshold levels (T levels) and maximum comfort levels (C levels) for each stimulation channel. Such data collection can be performed manually in a clinical setting by relying on subjective recipient responses, or by taking measurements directly from the cochlea such as by recording a neural response to stimulation in the manner set out in International Patent Publication No. WO 02/082982, the content of which is incorporated herein by reference. Users' T and C levels vary over time whether permanently such as in the case of tissue degradation, or temporarily for example in the case of the user having a short term disease. For optimal device function it is therefore important to clinically re-fit the device at regular intervals, however this is labor intensive.
Impedance measurements may also be made of the cochlea tissue. Existing impedance measurement techniques used in cochlear implant devices apply a stimulus with an electrode pair, and measure the voltage across that electrode pair. Such an impedance measurement provides information about the total impedance between the two stimulating electrodes, which can be useful to determine the functional condition of the implant. However, the voltage measured arises not only from the tissue impedance between the electrodes, but also from the electrode/tissue interface impedance at each electrode. The electrode/tissue interface impedance depends on the condition and the size of the electrode contact surface area and the amplitude of the stimulation current. The electrode/tissue interface impedance dominates and masks the tissue impedance. It is therefore difficult to precisely determine the tissue impedance and the biological condition of the tissue from such voltage measurements.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.